1. Field of the Invention
The present invention relates to ultrasonic measuring devices and in particular to an ultrasonic measuring device for use in monitoring the concentration of solvent vapor in the fixing station of a copying machine.
2. Description of the Prior Art
It is known from U.S. Pat. No. 3,049,810 to fix the toner images applied to a moving paper web in a fixing station of a printing or copying device by the use of solvent vapor. The vapor consisting of freon and methylene chloride dissolves the toner so that it can penetrate into the paper. The fixing station in which this process takes place may consist, for example, of a container or housing including an evaporation location by means of which the solvent is converted from a liquid into a vapor. The paper web is conducted through the housing and is exposed to the solvent vapor inside the housing. A condensation trap is commonly attached in front of the container opening for preventing the escape of solvent vapor into the environment.
A problem relating to the above known process and apparatus is that a perfect fixing of the toner images on the paper web can be achieved only when the solvent vapor has a specific concentration. It is therefore necessary that the concentration of the solvent vapor be determined within the fixing station by means of a measuring device.
Ultrasonic measuring devices operating on the principle that the transit time of an ultrasonic signal in a medium such as air varies depending upon the concentration of a vapor in the air are also known to those skilled in the art. One such ultrasonic measuring device is known, for example, from German AS No. 2,024,882 in which the electroacoustical transducer serves the function of transmitting the ultrasonic oscillation as well as receiving the oscillation which has been reflected by a reflector.
Ideally a device disposed in the fixing station of a printing or copying machine for providing an electronic signal corresponding to the concentration of the solvent vapor therein should provide a signal to an evaluation logic means connected to the sensor for determining whether the vapor concentration is within the specified range for proper operation, whether new solvent must be supplied and evaporated, or whether deviations from the specified value are so large that the printing or copying machine must be shut off.
A problem which is associated with any vapor concentration monitoring means which is used to control the operation of a device is that temporary disruptions in the vapor concentration may occur within the volume of air monitored by the sensor which may result in a changing of the density of the fixing agent vapor which are temporally and topically limited and therefore are not representative of the average density of the solvent vapor in the entire volume of the fixing station. Such disruptions may be caused by, for example, the formation of eddies due to spontaneous evaporation when the solvent is injected into the fixing station, thermal stratifications between the heated evaporation location and the condensation trap, and turbulences due to the moving paper web. These usually brief and spatially limited disruptions of the homogeneous distribution of the solvent vapor mixture have only a negligible influence on the fixing quality. The minimally different effect of the solvent in the vicinity of such temporary disruptions is balanced out over the longer fixing path.
Such disruptions do, however, significantly vitiate the measurements of the vapor concentration and the evaluation thereof in the limited volume of conventional detection devices. Diffractions of the ultrasonic wave may be caused which result in interference patterns, particularly due to the topical fluctuations of the vapor concentration. Such interference patterns may partially or entirely cancel the echo from the ultrasonic reflector or prevent the generation of useable electrical pulses at the receiver due to out of phase excitation of various portions of the sound-absorbing surface of the ultrasonic transducer.